The present invention relates generally to the field of gas analyzers. More particularly, the present invention relates to a technique for optical stabilization of temperature effects on an infrared gas analyzer.
Infrared (IR) gas analyzers typically include an infrared energy source and an infrared sensor. Disposed between the infrared energy source and the infrared sensor is a cell containing a gas mixture to be analyzed. A filter is generally carried on a rotatable wheel such that multiple filters can be rotated into position between the infrared energy source and infrared sensor. Infrared energy passes through the sample and is reduced by the presence of any gas that absorbs the infrared energy. Selectively imposing filters in the path of the beam of infrared energy changes the wavelength band of the infrared energy. Typically, each filter passes only radiation at a narrow band corresponding to a characteristic absorption wavelength band of a particular gas of interest. As such, through use of an infrared filter that is selected for each gas to be monitored in the gas sample, only infrared energy that can be absorbed by that gas is allowed to pass through the filter to be detected by the sensor.
In most IR gas analyzers, there is the need for some form of calibrating reference for the sensor. Conventional calibrating references include chopped dual beam, rotating filters or any other method that produces a varying optical energy signal on the sensor. FIG. 1 shows a graph 5 depicting a typical electrical output of a sensor in a conventional IR gas analyzer, where the low sensor data points (located at the xe2x88x921 level) represent the voltage measured by the sensor when the filter wheel or chopper is positioned at a dark time (i.e., when all of the IR energy from the IR source is blocked such that the sensor is dark). All measurements are referenced to this point. There is also a reference gas or reference filter that represents a constant attenuation of the IR source energy. These smaller peaks (represented by the 0.6 levels on graph 5), allow for the calibration of the larger peaks that have been (or will be) attenuated by the unknown gas to be measured. By subtracting the dark time from these two peak readings and taking the ratio of the known reference peak to the unknown gas peak, conventional gas analyzers obtain an absolute reading that is representative of the gas amount being measured.
FIG. 1 also shows the sensor signal square wave or xe2x80x9cflat topxe2x80x9d output. This output is typical of a dual path chopped or negative gas filter type gas analyzer. When the chopper or filter is in alignment with the IR source and sensor, the energy on the sensor is maximum and remains there until the chopper or filter goes out of alignment. The impingement of the maximum energy on the sensor during the alignment period causes the flat top on the signal wave form. This square wave in combination with the difference in peak levels with respect to the same level of the dark time means that, in order to maintain the electronic signal without distortion, the buffer amplifier must pass all frequency from DC to the 3 rd, 5th and possibly higher (e.g., 7th) harmonic of the fundamental. Thus, filtering the sensor noise now requires a balance in distortion and noise.
Ideally, in order to use conventional methods for determining gas amounts, the sensor should be linear. In practice, however, the best infrared sensors (in terms of cost versus speed versus sensitivity) are nonlinear. Nonlinear sensors used in gas analyzers are error prone in relative measurements due to the changing temperature of the sampling environment. To reduce the errors caused by the changing temperature and complex data signals, IR type gas analyzers typically use expensive sensor coolers and sampling environment heaters to reduce the effects of sensor data error.
Thus, there is a need to reduce gas reading errors in gas analyzers without using expensive heaters and coolers that are slow to come to temperature. Further, there is a need to use non-linear sensors in gas analyzers without the data measurement errors which result from changes in environmental temperature. Even further, there is a need to stabilize measurements which are typically unstable by simplifying the sensor data wave signal.
One embodiment of the invention relates to a method of stabilizing temperature effects on a gas analyzer. The method includes transmitting infrared energy through a plurality of cells in a reference cell drum to a sensor in the gas analyzer. The plurality of cells include at least one cell containing a reference gas and at least one cell containing an air reference. The method also includes interposing a filter between the bulb and the sensor in at least one of the plurality of cells in the reference cell drum, wherein energy from the infrared beam is reduced. The method further includes rotating the reference cell drum such that the beam of infrared energy fully passes through each of the plurality of cells for a limited period of time such that the sensor generates a substantial sine wave signal.
Another embodiment of the invention relates to a gas analyzer including a source which emits infrared energy, a sensor which detects infrared energy emitted from the source, a reference cell drum interposed between the source and the sensor, and at least one filter. The reference cell drum includes a plurality of cells through which the infrared energy passes from the source to the sensor. The plurality of cells includes at least one cell containing a reference gas and at least one cell containing an air reference. The reference cell drum is configured to rotate such that the infrared energy fully passes through each of the plurality of cells for a limited period of time such that the sensor generates a substantial sine wave signal. The at least one filter is located within the at least one cell containing an air reference. The at least one filter provides broad spectrum attenuation to reduce the infrared energy passing through the at least one cell containing an air reference.
Another embodiment of the invention relates to a gas analyzer which stabilizes temperature effects without using heaters and coolers. The gas analyzer includes means for transmitting infrared energy through a plurality of cells in a reference cell drum to a sensor. The cells include at least one cell containing a reference gas and at least one cell containing an air reference. The gas analyzer also includes means for filtering infrared energy passing through the at least one cell in the reference cell drum containing an air reference and means for rotating the reference cell drum such that the infrared energy fully passes through each of the plurality of cells for a limited period of time such that the sensor generates a substantial sine wave signal.
Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.